keyboard layout change in Xorg 7.2

[Novembre]

I have Xfce 4.4.1_1 on top of X.org 7.2 running on FreeBSD
6.2-RELEASEinstalled on a Pentium III-S
1.4GHz machine.
The problem is that the ALT keys are not working when using XkbLayout option
in xorg.conf when I put the following lines in /etc/X11/xorg.conf:

Option     "XkbLayout"     "us,ir"
Option     "XkbOptions"    "grp:alt_shift_toggle"

With these options, single ALT keys do not work, and I mean not just in
changing the keyboard layout, but anywhere! For example, ALT-TAB which
should change the current window does not work anymore. I have found out
that in this case, i.e. using XkbLayout option, if I press both ALT keys at
the same time, it'll do work! So, LEFTALT-RIGHTALT-TAB changes the current
window, and LEFTALT-RIGHTALT-SHIFT changes the layout. I have no idea why,
and I see no reason why, since I told X that the layout needs to be changed
when I press only ONE of the ALT keys (and I have not mentioned which one,
so both should be okay) and the SHIFT key. It's very troublesome, and I
don't want to hold down both ALTs instead of just one of them. The same
setting used to work when I was using Xfce 4.2 on top of X.org 6.9.
Any ideas?

Thanks a lot